The present invention relates to semiconductor laser devices and more particularly concerns a laser diode assembly allowing accurate positioning and alignment of individual laser bars in two-dimensional stacked arrays.
The numerous advances realized over the last decade in the design, fabrication and processing of high-power semiconductor laser sources, as well as in related technologies, have made these devices the preferred candidates as sources of light for an ever increasing variety of applications. These applications include the optical pumping of solid-state lasers, material processing, light illumination systems, remote sensing, and various medical applications such as in photodynamic therapy and in dermatology. High-power semiconductor laser devices are known in the form of elongated, thin laser diode bars comprising a plurality of individual laser emitters set along an axis parallel to the semiconductor PN junction plane of the emitters. FIG. 1 (prior art) illustrates a schematic drawing of a laser diode bar 10. The axis parallel to the semiconductor PN junction plane is usually referred to as the slow axis 12, while the orthogonal axis along the thickness of the laser diode bar is denoted as the fast axis 14. This terminology comes from the fact that the emitted laser beams 16 diverge strongly along the fast axis direction, thus requiring the use of fast optical elements for efficient collimation of the beams along this axis. The emitted laser beams 16 propagate along the Z direction 18 perpendicular to the plane of the front facet 20 of the laser bar 10. Common laser diode bars comprise typically 10 to 50 individual emitters 22, and the total width of the bars is generally set to 1 cm. Each individual emitter 22 has typical dimensions of 50-200 xcexcmxc3x971 xcexcm, and they are represented by the small filled rectangles in FIG. 1. The thickness of the schematic laser diode bar 10 as illustrated in FIG. 1 is greatly exaggerated since it scales in practice to about 1% of the width of the bar. The laser cavity length, defined as the spacing between both front 20 and rear 24 cleaved facets, is in the order of 500-1000 xcexcm. Laser diode bars made from the AlGaAs material system for emission of laser light in the 790-860 nm wavelength range can routinely emit tens of Watts of CW optical power. Although these output power levels suffice for many applications, the need for more powerful semiconductor laser sources is keenly felt for some highly-demanding applications that offer bright perspectives on the marketplace.
An obvious, but nevertheless efficient approach for increasing the total output power radiated by a semiconductor laser device is to stack several laser diode bars one above the other, thus creating a two-dimensional array of laser emitters. As it is the case for the laser beams escaping from the individual emitters of a laser diode bar, the various laser beams emitted from the stacked laser diode bars combine with each other in an incoherent manner to give an output beam carrying optical powers that can reach hundreds of Watts in CW regime and several kW (peak) during pulsed operation at low duty cycle. Because of their very small thickness, along with their elongated shape along the slow axis direction, it is readily seen that a lot of laser diode bars can be stacked along the fast axis direction without sacrificing too much of the outstanding compactness of semiconductor laser sources. In principle, any desired total output power radiated from such an array could be obtained simply by stacking a sufficient number of laser bars of known output power characteristics, and then driving them properly. However, because of the very small size of each laser bar combined with their typical wall-plug efficiency attaining around 40% in the best cases, each Watt of radiated optical output power is accompanied by about one Watt of wasted heat generated by the laser array. This heat must be properly removed away from the laser array, since most of the characteristics of interest of the radiated output beam are found to be sensitive to the local temperature increase. More importantly, the lifetime of the laser bars dramatically shortens when operated at an excessive temperature. The problem becomes particularly critical for laser bars stacked with a high density and operating CW or pulsed at high duty cycles. As a result, the selected design and method for stacking the laser bars into a two-dimensional array should promote efficient removal of the waste heat (low thermal impedance). In addition, each stacked laser diode bar must be driven with ideally the same current level in order to keep the output beam characteristics as uniform as possible over the whole emitting area of the laser array. Several approaches have been advanced for packaging laser diode bars into stacked arrays, most of them falling into two broad categories, well known in the art as the Rack-and-Stack and Bars-In-Grooves approaches, respectively.
In the Rack-and-Stack packaging technique, the individual laser diode bars are first mounted in submount assemblies comprising some support plates and/or spacer plates bonded together and made from various electrically conductive or insulating materials. The submount assemblies are then stacked one above the other and affixed to a heat sink or cooling means on their back side. The Rack-and-Stack technique is flexible, in the sense that the submount assemblies can be designed following various architectures. The submounts can be tested before final mounting, and some architectures allow replacement of defective submount assemblies without destroying the whole array. Using this method, high-density two-dimensional laser diode arrays with vertical pitch spacing as low as 0.4 mm (stacking density of 2.5 laser bars per mm of height) have been successfully realized. However, the labor-intensive steps required during manufacturing of these arrays as well as the need for a lot of small parts machined with high precision, make these arrays costly laser sources, thus prohibiting their use in applications wherein final cost is of primary concern. Furthermore, the high part count of these arrays along with the several soldering and bonding steps preclude collimation of the laser output beam using prefabricated microlens arrays, due to the great difficulties in positioning and aligning the mounted laser bars within the tight mechanical tolerances required for efficient and predictable beam collimation. Examples of Rack-and-Stack architectures for stacked laser diode arrays are disclosed in U.S. Pat. No. 4,716,568 (Scifres et al.), U.S. Pat. No. 4,719,631 (Conaway), U.S. Pat. No. 5,099,488 (Ahrabi et al.), U.S. Pat. No. 5,305,344 (Patel), U.S. Pat. No. 5,311,530 (Wagner et al.), U.S. Pat. No. 5,715,264 (Patel et al.), and U.S. Pat. No. 5,764,675 (Juhala).
The Bars-In-Grooves packaging architecture provides highly-integrated laser diode arrays since the individual laser diode bars are inserted and then soldered into precisely-spaced parallel grooves formed in an electrically insulating substrate material. Prior to mounting the laser bars into the grooves, the upper surface of the substrate and the side walls of the machined grooves must be metallized using techniques well known in the art, in order to ensure electrical continuity throughout the stack. The very low part count inherent to this technique promotes higher mechanical tolerances for the positioning and alignment of the laser diode bars while giving robust stacked laser arrays. In addition, removal of the waste heat is made easier since both metallized sides of the mounted laser bars are in close contact with the substrate, the latter being preferably made from a thermally conductive ceramic material like beryllium oxide (BeO). Compared to most of the Rack-and-Stack architectures, packaging laser diode arrays with the Bars-In-Grooves technique is somewhat easier, although the laser diode bars can easily be damaged during their loading into the grooves, whose width must fit closely (within a few microns) with the thickness of the bars. Deposition of uniform metallic coatings onto the side walls of grooves having typical thickness and depth of 150 xcexcm and 1 mm, respectively, is not a trivial task, and replacement of a defective bar after final mounting and soldering of the array is complicated, if at all possible. The Bars-In-Grooves technique is disclosed in U.S. Pat. Nos. 5,040,187; 5,128,951; 5,284,790 and 5,311,535, all to Karpinski. A laser diode array fabricated according to a slight variation of this technique and having a curved substrate suited for side pumping of cylindrical solid-state laser rods is disclosed in U.S. Pat. No. 5,627,850 (Irwin et al.).
Some other architectures of laser diode array assemblies that cannot be readily included in either of both basic categories discussed above are taught in the following U.S. Pat. No. 5,325,384 (Herb et al.), U.S. Pat. No. 5,394,426 (Joslin), U.S. Pat. No. 5,828,683 (Freitas), U.S. Pat. No. 5,835,515 (Huang), U.S. Pat. No. 5,835,518 (Mundinger et al.), and U.S. Pat. No. 5,909,458 (Freitas et al.).
In view of the above prior art, it seems that there is a need for a packaging architecture that could lead to the fabrication of affordable stacked laser diode arrays that may comprise a large number of laser diode bars.
It is therefore an object of the present invention to provide such a packaging architecture that can lead to the fabrication of affordable stacked laser diode arrays.
It is another object of the present invention to provide a packaging architecture allowing accurate registration of stacked laser diode arrays to prefabricated microlens arrays for efficient collimation of the laser output beam.
It is yet another object of the invention to provide a method of manufacturing a laser diode assembly in accordance with such an architecture.
Accordingly, the present invention provides a laser diode assembly including a substrate having a major front surface and a major back surface. At least one channel extends in the substrate from the major front surface thereof towards the back surface, and tapering inwardly. Each of the at least one channel has a back wall opposite the major front surface, a first side wall generally perpendicular to the major front surface, and a second side wall at an angle with respect to the first side wall.
The laser diode assembly further includes, for each of the at least one channel:
at least one metallic coating covering the major front surface and at least a portion of the first and second side walls;
a laser diode bar mounted in the channel adjacent to the first side wall;
at least one conductive body wedged between the laser diode bar and the second side wall; and
holding means for holding the laser diode bar and the conductive body inside the channel.
The present invention also provides a method for making a laser diode assembly. The method comprises steps of:
a) providing a substrate having a major front surface and a major back surface;
b) making at least one channel in said substrate from the major front surface thereof towards the back surface and tapering inwardly, said at least one channel having a back wall opposite the major front surface, a first side wall generally perpendicular to the major front surface, and a second side wall at an angle with respect to the first side wall;
c) covering the major front surface and at least a portion of the first and second side walls of each of the at least one channel with at least one metallic coating;
d) mounting a laser diode bar in each of the at least one channel, adjacent the first side wall thereof;
e) wedging at least one conductive body between the laser diode and the second side wall of each of the at least one channel; and
f) providing holding means for holding the laser diode bar and the conductive body inside the corresponding channel.
The present invention and its advantages will be better understood upon reading the following non restrictive description of preferred embodiments thereof, made with reference to the accompanying drawings.